The current annual global energy demand of ˜14 terawatt-years (TW-yrs) is expected to double by mid-century and triple by the end of the century. Such a large increase in energy demand cannot be met by the existing carbon-based technologies without further destabilizing the climate. The sun is the largest source of carbon-free energy (120,000 TW-yrs strike the planet's surface annually) and can be used to produce both electricity and fuel. Yet in the United States, solar electricity (e.g. photovoltaics) and solar-derived fuels (e.g. biomass) currently provide about 1 millionth of the total electricity supply and less than 0.1% of total energy consumption, respectively.
An area of great promise for low-cost solar energy conversion is inorganic thin-film photovoltaics (PV). Thin-film PV has the potential to revolutionize the photovoltaics industry via cheaper processing and eliminating the use of expensive silicon wafers that account for over 50% of total manufacturing cost of traditional silicon-based PV. Current thin-film technology is based on amorphous silicon, CdTe, and CIGS (copper indium gallium diselenide) as the active absorber layers. These materials can be made 50-100 times thinner than traditional silicon cells because of their larger optical absorption coefficients. The resulting lower cost per peak watt ($/Wp) is driving the extraordinary market growth of thin-film PV, which is projected to account for 28% of the solar market by 2012 (at $19.7 billion in sales). CdTe and CIGS are currently the most favored of the thin-film technologies due to their high laboratory cell efficiencies (16.5% for CdTe and 19.9% for CIGS) and because amorphous silicon encounters certain stability problems. However, the future market share and societal impact of CdTe and CIGS PV will be limited by the scarcity of tellurium (Te) and indium (I) in the Earth's crust. Most projections conclude that price constraints on tellurium and indium will limit CdTe and CIGS to 0.3 TWs or less of total solar conversion capacity, which falls far short of the tens of terawatts of carbon-free energy that are needed to meet the global energy challenge. To enable the rapid expansion of PV to the multi-TW scale, it is essential to develop alternative thin-film PV materials based on common (rock-forming) elements and inexpensive manufacturing processes.
Pyrite iron persulphide (β-FeS2, hereafter “pyrite”) is an under researched, extremely promising semiconductor for use as the active light-absorbing layer in thin-film PV. Pyrite offers a suitable bandgap (Eg=0.95 eV), strong light absorption (α>10−5 cm−1 for hv>1.3 eV), an adequate minority carrier diffusion length (100-1000 nm), and essentially infinite elemental availability, making it a particularly exciting material for multi-terawatt PV deployment. In principle, all of the United States' primary power demand (˜3.5 TW) can be met with 10% of the pyrite that is disposed of annually as mining waste in six U.S. states alone (even conservatively assuming a 5-micron thick pyrite active layer and 10% cell efficiency). One of pyrite's major strengths is that iron and sulfur will remain extremely cheap regardless of demand, even at multi-TW levels of PV deployment.
Iron pyrite is best described as Fe2+S22− in which the sulfur atoms are paired into persulfide dimers. The crystal structure is rock salt (space group Pa3) with an fcc sublattice of Fe2+ ions and sulfur dumbbells pointed along the <111> directions occupying the anion sites. This arrangement results in a slightly distorted octahedral coordination for Fe2+, which exists in its diamagnetic d6 configuration, and tetrahedral coordination of each sulfur atom to three iron ions and its dimer partner. Four formula units make up the unit cell. The basic electronic structure of pyrite has been the subject of extensive experimental and theoretical studies. The top of the valence band is formed by the overlap of nonbonding Fe 3d t2g orbitals, while the bottom of the conduction band is mostly Fe eg* states, with some hybridization of S 3p orbitals at higher energy.
Pyrite thin films have been prepared by many techniques, including the sulfurization of iron thin films, direct sputtering of FeS2, flash evaporation, electrodeposition, spray pyrolysis, and chemical vapor deposition (CVD). Pyrite thin films can be doped p-type with P, As or Sb, and n-type with Co, Cl or Br.
The research group of Helmut Tributsch at the Hahn-Meitner Institüt (now the Helmholtz Centre for Materials and Energy) investigated pyrite intensively for PV and photoelectrochemical cells beginning in 1983. Much of what is known about the synthesis and basic materials properties of pyrite for solar applications originates from the pioneering work of this laboratory and its collaborators. The Tributsch group developed several techniques for the preparation of single crystals and thin pyrite layers, studied the photoelectrochemistry of natural and synthetic samples, and reported the first pyrite solar cells in 1984. These initial devices were based on Schottky junctions with certain metals or liquid electrolytes and showed large photocurrents, small photovoltages, and efficiencies of ˜1%. Subsequent progress in improving the efficiency of pyrite devices has been extremely modest. In 1990, Tributsch reported a 2.8% efficient photoelectrochemical cell using an n-FeS2 single crystal in an aqueous iodide/triiodide electrolyte. See Ennaoui, A. et. al., “World Renewable Energy Congress. Energy and the Environment,” Ed. Sayigh, A. A. M. (Pergamon Press, Oxford, 1990) p. 458. Shortly thereafter, Antonucci found that heat treating pyrite in H2 produced a photoelectrochemical device with a higher photovoltage (460 mV), respectable current, and an efficiency as high as 3.3%; though suggestive that annealing treatments can boost pyrite VOC, nothing resulted from this work over the following twenty years. See Antonuccia, V., et. al., “Photoactive Screen-Printed Pyrite Anodes for Electrochemical Photovoltaic Cells,” Solar Cells 1991, 31, 119-141. The only other devices of note are pyrite-sensitized TiO2 nanocrystal photoelectrochemical cells reported in 1992 and p-n homojunction cells described in a thesis in 1995. See Ennaoui, A., et. al., “Photoelectrochemical Energy Conversion Obtained With Ultrathin Organo-Metallic-Chemical-Vapor-Deposition Layer of FeS2 (Pyrite) on TiO2,” Journal of the Electrochemical Society 1992, 139, 2514-2518; see also Blenk, O., “Fabrication and Characterization of FeS2 (Pyrite) for Photovoltaic Applications,” Ph.D. Thesis, University of Konstanz, Germany, 1995.
The major limitation on the conversion efficiency of pyrite cells is the low open-circuit voltage, which typically does not exceed 200 mV (˜20% of the band gap) at room temperature. This low photovoltage is blamed on band gap states created by sulfur vacancies in the bulk and at the surface of the material. Pyrite commonly exhibits a sulfur deficit of up to several percent. The defects are thought to be Schottky vacancies distributed homogeneously within the crystal. Sulfur deficiency lowers the iron coordination number from six to five and reduces the point-group symmetry of the iron coordination polyhedron from Oh to C4v. This change in the ligand field causes one of the Fe eg* states to relax into the forbidden gap, where it can act as a deep trap for carriers. These iron-derived mid-gap states are an intrinsic feature of pyrite surfaces on which Fe—S bonds have been cleaved (e.g., 100 and 111 surfaces).
The effect of a particular sulfur vacancy depends on whether it occurs in the bulk or at the crystal surface. The high quantum yield of sulfur-deficient pyrite suggests that bulk vacancies do not necessarily act as efficient recombination centers. Nevertheless, a sufficiently high density of bulk vacancies will give rise to a defect band that decreases the band gap, and thus the photovoltage, of the material. Bulk sulfur vacancies also appear to act as dopants in pyrite. Relative to bulk defects, surface sulfur vacancies seem to have a more deleterious effect on the performance of pyrite devices. The large concentration of five-coordinate iron ions at the surface necessarily creates a high density of mid-gap surface states, and these states limit the photovoltage by narrowing the surface band gap, pinning the Fermi level and acting as recombination centers. Sulfur vacancies at the crystal surface also lead to FeS-like layers (which are quasi-metallic), oxides, and other defects that introduce additional traps and recombination centers, increase the dark current, and further reduce the photovoltage of pyrite samples.
Surface states are especially important for pyrite because carriers are generated close to the surface due to its very large optical absorption coefficient. The complexity of pyrite defect chemistry, combined with the low level of funding devoted to this material since the first demonstration of pyrite solar cells, explains why pyrite has not attained a more advanced level of development as a practical material for solar energy conversion, despite its great promise. Therefore, Pyrite-based devices with enhanced photovoltage and efficiency are desirable.